This invention relates to novel catalysts for the selective vapor phase oxidation of a hydrocarbon to a dicarboxylic anhydride and processes for the manufacture of these catalysts.
Maleic anhydride and other dicarboxylic anhydrides are commercially manufactured by vapor phase oxidation of a hydrocarbon such as n-butane as the hydrocarbon flows through a fixed bed reactor containing vanadium-phosphorus oxide catalyst. During the process, acrylic acid, acetic acid, carbon monoxide, and carbon dioxide are formed as by-products of the reaction. In butane-based maleic anhydride plants, downstream process equipment such as heat exchangers and distillation trays from the fixed bed reactor are often fouled after prolonged operation with a polyacrylic polymer derived from accumulation and reaction of acrylic acid. Such fouling is particularly acute in plants operating with aqueous-based maleic anhydride recovery processes where the acrylic acid polymerizes and obstructs distillation trays, reducing the performance of the distillation column. There has been a need for an improved catalyst that inhibits the formation of acrylic acid by-product while preserving maleic anhydride yield.
Procedures for the preparation of catalysts comprised of vanadium-phosphorus oxide are disclosed and taught in the prior art. Many references teach that it is preferable to reduce the vanadium to the tetravalent state in solution. Such catalysts are prepared by contacting vanadium and phosphorus compounds under conditions where substantial reduction to the tetravalent state of vanadium occurs to form the catalyst precursor. In many instances, promoter elements are also included in the catalyst precursor. The catalyst precursor is recovered, dried, and calcined to produce the catalyst.
It is generally recognized that the most desirable catalysts contain a substantial proportion of vanadyl pyrophosphate, a compound represented by the formula (VO).sub.2 P.sub.2 O.sub.7 whose X-ray diffraction pattern exhibits primary peaks at d-spacings of 3.86, 3.14 and 2.98 .ANG.. Numerous references in the patent and open literature also discuss the incorporation of various metal ions as promoters in vanadium phosphorus oxide catalysts. A very wide variety of metal ions has been proposed for such purpose, and data have been published indicating a beneficial effect on yields or selectivity. Most of the literature does not speculate on how the promoters operate to enhance catalyst performance. However, some theories have been advanced. A survey article by Hutchings, "Effect of Promoters and Reactant Concentration on the Selective Oxidation of n-Butane to Maleic Anhydride Using Vanadium Phosphorus Oxide Catalysts," Applied Catalysis 72(1991), pp. 1-32 suggests that promoters serve a two fold function: to enable the formation of required vanadium phosphorus oxide ("VPO") compounds while decreasing the formation of deleterious phases; and to enable the formation of solid solutions which regulate the catalytic activity of the solid phase. Hutchings reports that promoter to vanadium molar ratios of 0.15 to 0.20 are considered to provide optimum results, though one study is adverted to in which favorable results were obtained with an Mo/V ratio of 0.04.
The Hutchings article further notes that the impregnation of a VPO catalyst with a promoter may increase the surface area. However, this reference does not mention, or indicate any significance to, the point at which surface area is measured. Nor does it suggest an effect of a promoter on the surface area of the catalyst. Most of the article concentrates on the reported effect of various promoters on the specific activity of a catalyst of a given area.
U.S. Pat. No. 4,043,943 describes vanadium-phosphorus oxides isolated from organic solutions which may contain minor amounts of molybdenum as a promoter. Molybdenum oxides are added at the 0.05 to 0.10 level and increase the surface areas of the catalysts after equilibration in a butane-oxygen gas stream.
U.S. Pat. Nos. 4,056,487, 4,105,586, 4,152,338, 4,152,339, 4,153,577, 4,158,671, 4,167,516 and 4,202,826 disclose catalyst disclose VPO catalyst complexes with several modifier elements selected from Mo, Cu, Ni, Co, Cr, Hf, Nb, Li, Re, Ru, Ba, Nd, Cb, Ce, and Y prepared from aqueous hydrochloric acid solution to which various amounts of oxalic acid and isopropyl alcohol are added. The ratio of molybdenum is 0.0025-0.04 mole per mole vanadium. Catalysts having a 0.010-0.012 Mo/V molar ratio were reported. Catalysts of this invention are said to produce molar yields of maleic anhydride from n-butane from 28.0 to 57.4% with salt bath temperatures from 388 to 450.degree. C.
U.S. Pat. No. 4,065,468 describes a process for the production of maleic anhydride from n-butane using a catalyst that contains the oxides of vanadium, phosphorus, molybdenum, antimony, and various promoter elements such as nickel, bismuth, and titanium. Catalysts are reported having a Mo/V molar ratio of 2-12.1 m.sup.2 /g.
U.S. Pat. Nos. 4,110,350 and 4,177,161 give a process for oxidizing unsaturated hydrocarbons having 4 to 6 carbon atoms to maleic anhydride using a catalyst comprised of vanadium-phosphorus oxides and titanium with optional other elements including molybdenum in a Mo/V molar ratio of 0-1. The aqueous-prepared catalysts are said to produce yields of around 66% to maleic anhydride from 1,3-butadiene at 100% conversion and 450.degree. C. One such reported catalyst had a molar ratio of Mo/V of 0.1.
U.S. Pat. No. 4,147,661 describes VPO catalysts modified with molybdenum in an atomic Mo/V ratio of 0.005-0.5 as prepared in an isobutanol and hydrogen chloride gas medium. Catalysts having an atomic Mo/V ratio of 0.03-0.05 are reported as providing a surface area of 16-24 m.sup.2 /g for the calcined catalyst.
U.S. Pat. Nos. 4,151,116 and 4,244,878 describe catalysts containing vanadium-phosphorus oxide and a post-deposited promoter selected from elements that include molybdenum in a Mo/V atomic ratio of 0.12. All catalysts and their promoters are made in aqueous hydrochloric acid. Reported molar yields of maleic anhydride for the catalysts of this invention range from 32.0 to 48.8% with selectivities of 40.0 to 61.0% and salt bath temperatures of 410 to 500.degree. C.
U.S. Pat. Nos. 4,172,084, 4,218,382, 4,219,484, and 4,225,465 describe catalysts of vanadium, phosphorus and uranium formed from aqueous oxide slurries that include promoter elements such as molybdenum in a Mo/V atomic ratio of 0.01-5, preferably 0.05.
U.S. Pat. Nos. 4,209,423 and 4,288,372 disclose catalysts containing vanadium, phosphorus and promoters such as molybdenum which are prepared from hydrochloric acid solutions. The molar ratio of molybdenum to vanadium ranges from 0.0015 to 1. A catalyst precursor prepared at a Mo/V ratio of 0.082 is reported to have a Mo/V atomic ratio of 0.043 in the dried solid. The final catalyst is reported to have a surface area of 9 m.sup.2 /g and gave a 55% molar pass yield of maleic anhydride at an n-butane conversion of 75%, 420.degree. C., 1000/hr. of 1.5% n-butane in are at atmospheric pressure.
U.S. Pat. Nos. 4,222,945 and 4,317,777 describe a catalyst prepared by forming a VPO precursor in an aqueous hydrochloric acid medium, drying the solid precursor, boiling the solid to form a suspension, filtering the solid from the suspension, drying the solid, and mixing the solid with molybdenum trioxide at a Mo/V atomic ratio of 0.025. The resulting solid is ball milled, pelleted, crushed and sieved to give 500-700.mu. particles prior to calcination at 385.degree. C. in a 1.5% n-butane/air mixture at 1000/hr GHSV. The final catalyst is reported to have a surface area of 32 m.sup.2 /g.
U.S. Pat. Nos. 4,396,535 and 4,465,846 disclose vanadium-phosphorus oxide catalyst precursors prepared in an aqueous slurry which are dried, crushed, slurried in alcohol, filtered, dried and calcined. Promoters such as molybdenum can be added to the precursor after drying or to the catalyst after calcination.
U.S. Pat. No. 4,400,522 describes vanadium-phosphorus oxide catalysts prepared from isobutanol with the addition of various promoter elements including molybdenum at a promoter to vanadium ratio of 0-0.5.
U.S. Pat. Nos. 4,416,802, 4,416,803, 4,418,003 4,510,259 and 4,748,140 describe VPO catalyst modified with a cometal such as molybdenum in a molar Mo/V ratio of 0.001-0.4. The catalysts are prepared from an aqueous hydrochloric acid solution. Catalysts having a molar Mo/V ratio of 0.03 are reported.
U.S. Pat. No. 4,596,878 discloses a method for regeneration of VPO catalysts which may contain molybdenum in a molar Mo/V ratio of 0.001-0.4.
U.S. Pat. No. 4,649,205 discloses a process for the reactivation of a vanadium-phosphorus oxide catalyst promoted by metals from a group that includes molybdenum in an atomic ratio of Mo to V of 0.005 to 0.25. The catalysts benefit by the use of water/trialkylphosphate mixtures fed to the reactor with n-butane/air. Molar yields of 47.9 to 54.4%. maleic anhydride are disclosed for catalysts having a Mo/V ratio of 0.03 with per day productivities ranging from 0.79 to 1.76 lb.-maleic anhydride/lb.-catalyst.
U.S. Pat. No. 4,732,885 describes a process for the manufacture of vanadium-phosphorus oxide (VPO) catalysts using a cometal as a promoter from a group that includes molybdenum. The cometal/vanadium ratio may range from 0.001 to 0.4. The promoted catalysts are used in the production of maleic anhydride from n-butane. The catalyst is prepared from an organic ether solvent containing a phosphoryl chloride in the presence of water or an aliphatic alcohol. The catalyst is activated in a gas stream of n-butane and water. Molybdenum-promoted VPO catalyst having a Mo/V molar ratio of 0.03 reportedly displayed a yield ranging from 77 to 103% maleic anhydride when a 1.1 to 1.5% n-butane gas stream was passed over the catalyst at a temperature between 375 and 440.degree. C. and 1200/hr. to 2000/hr. space velocity. The surface area of one of the molybdenum-promoted catalysts was reportedly 40 m.sup.2 /g.
U.S. Pat. No. 4,824,819 discloses catalysts for the production of maleic anhydride which are comprised of vanadium-phosphorus oxide and a cometal such as molybdenum. A vanadium compound is deposited on top of the PO/cometal composition to give the final catalyst. Catalysts of this invention having a Mo/V molar ratio of 0.03 are described as exhibiting 72-89% conversion of a 1.1% n-butane/air feed at 1200/hr. space velocity at 431-445.degree. C. with 80-94% molar yield of maleic anhydride.
U.S. Pat. No. 4,950,769 describes a process for vapor phase oxidation of n-butane to maleic anhydride by a vanadium-phosphorus oxide catalyst containing a cometal selected from a group that includes molybdenum in a Mo/V molar ratio of 0.001 to 0.2. A peroxide is added to the reactor gas stream to improve the catalyst operation. A molybdenum-promoted catalyst was reportedly prepared by an aqueous process wherein the Mo/V molar ratio was about 0.03.
U.S. Pat. Nos. 4,933,312, 4,957,894, 4,965,235, 4,996,179, 5,093,298, and 5,095,125 disclose processes for the manufacture of catalysts used for maleic anhydride production. The catalysts are comprised of vanadium-phosphorus oxide and contain a cometal as a promoter from a group that includes molybdenum at a Mo/V molar ratio of 0.001-0.2. Catalysts prepared by the aqueous processes of the invention do not exhibit more than 2% expansion when activated for the oxidation of n-butane to make maleic anhydride. Catalysts having a Mo/V molar ratio of 0.03 are reported as producing molar yields of maleic anhydride around 53% for a 1.1% n-butane stream passing over the catalyst at 1200/hr. space velocity.
U.S. Pat. Nos. 5,011,945, 5,019,545, and 5,134,106 describe VPO catalysts containing a modifier such as molybdenum in a molar Mo/V ratio of 0.001-0.2. Catalysts prepared by the reaction of a vanadium compound and a phosphoryl halide in an aqueous medium are reported which have a molar Mo/V ratio of 0.06.
U.S. Pat. Nos. 5,070,060, 5,158,923 and 5,262,548 describe VPO catalysts containing Li, Zn and Mo promoters prepared in an aqueous hydrochloric acid solution. The molar Mo/V ratio ranges from 0.005-0.025. The reported catalysts having a Mo/V molar ratio of 0.0065, 0.013 or 0.026 provide a surface area of 3.9-11.4 m.sup.2 /g for the tableted catalyst.
U.S. Pat. Nos. 5,280,003 and 5,296,436 disclose VPO catalysts promoted with Li, Zn and Mo prepared in anhydrous isobutanol medium wherein the Mo/V ratio ranges from 0.005-0.025. Catalysts having a molar Mo/V ratio of 0.013-0.016 are reported.
Thus, it has long been known that various promoters including molybdenum can be added to VPO catalysts to obtain improved performance. Numerous patents from the 1980s as described above disclose the benefit of molybdenum addition in preparing aqueous derived catalysts. Since then, VPO catalysts prepared in aqueous media have been outpaced by catalysts prepared in non-aqueous media. See the Hutchings article discussed above. As indicated by the above cited patents, molybdenum has been suggested for VPO catalysts prepared in non-aqueous systems, in optimal Mo/V ratios of about 0.01-0.05.